In the field of continuous casting, there is a strong demand for the development of a technique for manufacturing directly from a molten metal a thin cast strip having a shape generally equal to a finished shape of sheet metal for the purpose of reducing the manufacturing cost, creating new metals, and so on. To meet this demand, various methods, for example, a twin-drum method using a pair of cooling drums with internal water-cooling mechanisms, a single drum method using one cooling drum and a drum-belt method based on the formation of a meniscus between a cooling drum and a belt, have been proposed and some of them have been improved to such a level as to be industrially practicable.
In these continuous casting methods, it is important to stably maintain surface characteristics of a thin cast strip at a high level. That is, continuous casting methods of this kind have been developed with the aim of obtaining a thin cast strip which is capable of reducing the rate of rolling in a subsequent step unlike slabs manufactured by the conventional continuous casting facilities. Therefore, if there is a variation in the thickness of a thin cast strip, a crack in the surface thereof, or the like, it would remain as a defect in the surface of the product and there is a considerable risk of impairing the value of the product on the market.
Various methods have therefore been studied and proposed to stably manufacture thin cast strips having good surface qualities. For example, a method of forming irregularities in the peripheral surface of a cooling drum to form air gaps between a cast metal and the cooling drum has been proposed in Japanese Unexamined Patent Publication No. 60-184449. The heat removing capacity of the cooling drum is reduced by the air gaps so that the molten metal is slowly cooled. Consequently, the thickness of the solidified shell is made uniform in the widthwise direction of the shell and it is possible to manufacture a thin cast strip having improved shape characteristics.
However, if a thin cast strip is manufactured by using a cooling drum having surface irregularities or dimples formed in its peripheral surface, transverse creases may be easily formed in the surface of the obtained cast strip. The cause of this phenomenon will be described below with reference to the drawings.
FIG. 5 shows a meniscus portion of one of two cooling drums of a twin drum type continuous casting machine. The same problem is encountered in the case of the single drum process or the drum-belt process.
Molten metal 4 injected into a basin 3 is vibrated by surface irregularities or dimples 2 formed in the peripheral surface of a cooling drum 1 to cause surface waves 5. As the cooling drum 1 is rotated in the direction of arrow A, the molten metal 4 is cooled by the peripheral surface of the cooling drum 1 to form a solidified shell which is fed into the drum gap.
If surface waves 5 occur in the molten metal surface during this process, the state of contact between the molten metal 4 and the peripheral surface of the cooling drum 1 is deteriorated, so that irregular gaps are easily formed at the interface. These gaps act to cause a reduction in the rate of heat transfer to the cooling drum 1, causing irregular changes in the growing speed of the solidified shell and the cooling speed in the lengthwise direction of the cast strip.
Transverse creases are therefore formed in the resulting thin cast strip. Also, even if the generation of transverse creases can be avoided, the heat history of the thin cast strip is locally changed since the conditions of the growth and cooling of the solidified shell are varied by surface waves 5.
Variations in the heat history caused in this manner result in non-uniformity of the gloss of the product surface and a coarse crystalline structure and so on and considerably reduce the value of the product on the market particularly in the case of a stainless steel thin plate or the like whose surface characteristics is considered to be particularly important. It has been found that the diameter and the depth of dimples 2 and the distance between the centers thereof relate greatly to surface waves 5 and that an effect of suppressing surface wave 5 can be obtained by reducing the diameter, increasing the depth and reducing the distance.
Detailed studies made by the inventors have revealed that the occurrence of surface waves can be suppressed by eliminating the periodicity of the dimple distance, for example, by irregularly setting the distance between the dimples. In this case, analysis of the distribution density of the dimple positions is effective as a method for confirming the irregularity. The distribution density referred to herein is the density of probability of the existence of dimples in immediate proximity to any dimple as defined by setting X-axis in the drum axial direction and Y-axis in the drum circumferential direction on the drum peripheral surface, as shown in FIG. 7, and by plotting the difference (X, Y) between arbitrary dimple position (Xi, Yi) and the position (Xj, Yj) of a dimple in immediate proximity thereto with respect to .THETA.=Tan.sup.-1 (X/Y). FIG. 8 shows results of this analysis. In contrast, if there are regularities, different results are obtained from different kinds of regularities as discussed below. FIG. 9 shows an analysis result from case where there is a periodicity in the drum surface axial direction, and FIG. 10 shows an analysis result from a case where there is a periodicity in the drum circumferential direction.
However, dimples 2 of this kind have ordinarily been formed by etching and it has therefore been difficult to set a small dimple diameter and a large dimple depth simultaneously. Also, it has been practically impossible to freely change the dimple distance because of large burdens in terms of time and cost.